Thin film metal silicides and methods for formation

ABSTRACT

The disclosed subject matter provides thin films including a metal silicide and methods for forming such films. The disclosed subject matter can provide techniques for tailoring the electronic structure of metal thin films to produce desirable properties. In example embodiments, the metal silicide can comprise a platinum silicide, such as for example, PtSi, Pt 2 Si, or Pt 3 Si. For example, the disclosed subject matter provides methods which include identifying a desired phase of a metal silicide, providing a substrate, depositing at least two film layers on the substrate which include a first layer including amorphous silicon and a second layer including metal contacting the first layer, and annealing the two film layers to form a metal silicide. Methods can be at least one of a source-limited method and a kinetically-limited method. The film layers can be deposited on the substrate using techniques known in the art including, for example, sputter depositing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 15/007,867, filed on Jan. 27, 2016, which claims priority toU.S. Provisional Application No. 62/112,579, filed on Feb. 5, 2015, theentire contents of all of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under grant numberCMMI1334241 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

BACKGROUND

Solid-state reactions to form metal silicides (Me_(x)Si) can be used toform ohmic contacts, Schottky barrier contacts, gate electrodes, localinterconnects, and diffusion barriers in the microelectronics industry.Certain metal silicides, i.e., platinum silicides (Pt_(x)Si), candemonstrate certain characteristics useful for contacts for nanoelectromechanical (NEM) switches. For example, platinum silicides candemonstrate wear and oxidation resistance during nanoscale mechanicalcontact.

Certain Pt_(x)Si can be formed from thin film Pt deposited on a singlecrystal Si (sc-Si) wafer using a two part process. First, Pt atomsdiffuse into the sc-Si and a Pt-rich silicide, Pt₂Si, is formed. Afterthe Pt is consumed, atoms from the sc-Si can diffuse into Pt₂Si to formthe thermodynamically stable PtSi phase. However, certain NEM switchgeometries can demand silicidation away from the sc-Si carrier wafer,leading to an incompatibility of the sc-Si/Pt silicidation process withsuch NEM switch geometries.

SUMMARY

The disclosed subject matter provides thin films including a metalsilicide and methods for forming such films. In example embodiments, thedisclosed subject matter provides methods which include providing asubstrate, depositing at least two film layers on the substrate whichinclude a first layer including amorphous silicon and a second layerincluding metal contacting the first layer, and annealing the two filmlayers to form a metal silicide. The method can be at least one of asource-limited method and a kinetically-limited method. The film layerscan be deposited on the substrate using any thin film depositiontechnique known in the art including, for example, sputter depositing.The film layers can be deposited on the substrate by first depositingthe first layer and subsequently depositing the second layer on top ofthe first layer.

In certain embodiments, the substrate can include silicon. A diffusionbarrier can be deposited between the substrate and the at least two thinfilm layers. The diffusion barrier can include at least one of aluminumnitride and silicon nitride. The metal can include at least one ofplatinum, titanium, nickel, tungsten, cobalt, molybdenum, and chromium.The metal silicide can be a platinum silicide. For example, the metalsilicide can include Pt₃Si. The platinum silicide can be at least about40% Pt₃Si, at least about 45% Pt₃Si, at least about 50% Pt₃Si, at leastabout 55% Pt₃Si, at least about 60% Pt₃Si, at least about 65% Pt₃Si, atleast about 70% Pt₃Si, at least about 74% Pt₃Si, at least about 75%Pt₃Si, at least about 80% Pt₃Si, at least about 85% Pt₃Si, and at leastabout 88% Pt₃Si.

In another example embodiment, the disclosed subject matter provides asource-limited method for forming a thin film including a metalsilicide. The method can include identifying a desired phase of a metalsilicide, determining a ratio of an amount of a metal to an amount ofamorphous silicon based on the desired phase of the metal silicide,providing a substrate, depositing at least two film layers on thesubstrate including a first layer of amorphous silicon and a secondmetal layer contacting the first layer, the at least two film layersincluding the amorphous silicon and the metal in amounts based on thedetermined ratio, and annealing the at least two film layers to form thedesired phase of the metal silicide. The ratio can be determined, forexample, by experimentally determining the ratio or calculating theratio.

In accordance with embodiments of the disclosed subject matter, thesubstrate can include a wafer. A diffusion barrier can be depositedbetween the substrate and the at least two thin film layers. Thediffusion barrier can include at least one of aluminum nitride andsilicon nitride. The metal can include at least one of platinum,titanium, nickel, tungsten, cobalt, molybdenum, and chromium. Inaccordance with an exemplary embodiment of the disclosed subject matter,the metal silicide can be a platinum silicide.

The disclosed subject matter also provides kinetically-limited methodsfor forming a thin film. In one example, the method includes identifyinga desired phase of a metal silicide, determining a time-temperatureregime based on the desired phase, providing a substrate, depositing atleast two film layers on the substrate, the at least two film layersincluding a first layer including amorphous silicon and a second layerincluding metal contacting the first layer, and annealing the at leasttwo film layers using the determined time-temperature regime to form thedesired phase of the metal silicide. The time-temperature can bedetermined, for example, by experimentally determining thetime-temperature regime.

In accordance with yet another aspect, the disclosed subject matterprovides a thin film including a metal silicide. In accordance with oneembodiment, a thin film is formed by a process including providing asubstrate, depositing at least two film layers on the substrate, the atleast two film layers including a first layer including amorphoussilicon and a second layer including metal contacting the first layer,and annealing the at least two film layers to form a metal silicide.

In accordance with another embodiment, a thin film is formed by aprocess including identifying a desired phase of a metal silicide,determining a ratio of an amount of a metal to an amount of amorphoussilicon based on the desired phase of the metal silicide, providing asubstrate, depositing at least two film layers on the substrate, the atleast two film layers comprising a first layer including amorphoussilicon and a second layer including metal contacting the first layer,the at least two film layers including the amorphous silicon and themetal in amounts based on the determined ratio, and annealing the atleast two film layers to form the desired phase of the metal silicide.

In accordance with a further embodiment, a thin film is formed by aprocess including identifying a desired phase of a metal silicide,determining a time-temperature regime based on the desired phase,providing a substrate, depositing at least two film layers on thesubstrate, the at least two film layers including a first layerincluding amorphous silicon and a second layer including metalcontacting the first layer, and annealing the at least two film layersusing the determined time-temperature regime to form the desired phaseof the metal silicide.

In a further aspect, the disclosed subject matter provides a thin filmincluding Pt₃Si. The thin film can be at least 40% Pt₃Si, at least 45%Pt₃Si, at least 50% Pt₃Si, at least 55% Pt₃Si, at least 60% Pt₃Si, atleast 65% Pt₃Si, at least 70% Pt₃Si, at least 74% Pt₃Si, at least about75% Pt₃Si, at least about 80% Pt₃Si, at least about 85% Pt₃Si, and atleast about 88% Pt₃Si.

In another aspect, the disclosed subject matter provides systems anddevices including thin films in accordance with the disclosed subjectmatter. The device can be, for example, a nanoeletromechanical switch, ajet engine, a plasmonic device, a battery such as a lithium-ion battery,or a field emitter.

In yet another aspect, the disclosed subject matter provides a methodfor forming a thin film including a metal silicide. The method caninclude providing a first layer of amorphous silicon and a second layerof mater, and diffusing the amorphous silicon into the metal. Thediffusing can include annealing the first layer of amorphous silicon andthe second layer of metal to diffuse the amorphous silicon into themetal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary embodiment of a method for forming athin film comprising a metal silicide in accordance with the disclosedsubject matter.

FIG. 2A illustrates a cross-sectional view of an as-deposited sampleusing an exemplary embodiment of a method for forming a thin filmcomprising a metal silicide in accordance with the disclosed subjectmatter.

FIG. 2B illustrates a perspective view of an as-deposited sample usingan exemplary embodiment of a method for forming a thin film comprising ametal silicide in accordance with the disclosed subject matter.

FIG. 3A illustrates a cross-sectional view of an annealed sample usingan exemplary embodiment of a method for forming a thin film comprising ametal silicide in accordance with the disclosed subject matter.

FIG. 3B illustrates a perspective view of an annealed sample using anexemplary embodiment of a method for forming a thin film comprising ametal silicide in accordance with the disclosed subject matter.

FIG. 4 illustrates an exemplary embodiment of a source-limited methodfor forming a thin film comprising a metal silicide in accordance withthe disclosed subject matter.

FIG. 5 illustrates an exemplary embodiment of a kinetically-limitedmethod for forming a thin film comprising a metal silicide in accordancewith the disclosed subject matter.

FIGS. 6A and 6B illustrate source-limited formation of Pt_(x)Si inaccordance with an exemplary embodiment of the disclosed subject matter.FIG. 6A shows electron diffraction patterns of as-deposited Pt, andPt₃Si, Pt₂Si, and PtSi films produced using a source-limited solid-statediffusion approach in accordance with an exemplary embodiment of thedisclosed subject matter. Gray slices indicates the theoreticaldiffraction ring positions according to the theoretical crystalstructure schematically shown for each phase. FIG. 6B shows a conceptualschematic of the source-limited solid-state diffusion process inaccordance with an exemplary embodiment of the disclosed subject matter,which utilizes precise control of the Pt and a-Si precursor filmthickness to predetermine the desired Pt_(x)Si phase.

FIGS. 7A-7D show surface characterization of as-deposited Pt and formedPt_(x)Si on TEM grids using source-limited solid-state diffusion inaccordance with an exemplary embodiment of the disclosed subject matter.FIG. 7A shows the high-resolution XPS spectra of the Pt4f peaks of thePtSi-rich sample (Pt:a-Si=1:3). Quantitative analysis of the XPS resultsindicated the formation of 92% of PtSi. FIG. 7B shows thehigh-resolution XPS spectra of the Pt4f peaks of the Pt₂Si-rich sample(Pt:a-Si=1:1). Quantitative analysis of the XPS results indicated theformation of 90% of Pt₂Si. FIG. 7C shows the high-resolution XPS spectraof the Pt4f peaks of the Pt₃Si-rich sample (Pt:a-Si=1:2). Quantitativeanalysis of the XPS results indicated the formation of 74% of Pt₃Si.FIG. 7D shows the high-resolution XPS spectra of the Pt4f peaks of theas-deposited sample Quantitative analysis of the XPS results indicatesthe presence of pure metallic Pt.

FIGS. 8A and 8B illustrate kinetically-limited formation of Pt_(x)Si inaccordance with an exemplary embodiment of the disclosed subject matter.FIG. 8A shows electron diffraction patterns of as-deposited Pt andPt₃Si, Pt₂Si, and PtSi films produced using a kinetically-limitedsolid-state diffusion approach in accordance with an exemplaryembodiment of the disclosed subject matter. Gray slices indicates thetheoretical diffraction ring positions according to the theoreticalcrystal structure schematically shown for each phase. FIG. 8B shows aconceptual schematic of the kinetically-limited solid-state diffusionprocess in accordance with an exemplary embodiment of the disclosedsubject matter, which utilizes precise control of the temperature-timeregime to obtain the desired PtSi phase.

FIGS. 9A-9D show surface characterization of as-deposited Pt and formedPt_(x)Si on TEM grids using kinetically-limited solid-state diffusion inaccordance with an exemplary embodiment of the disclosed subject matter.FIG. 9A shows the high-resolution X-ray photoelectron spectroscopy (XPS)spectra of the Pt4f peaks of the PtSi-rich sample. Quantitative analysisof the XPS results indicated the formation of 91% of PtSi. FIG. 9B showsthe high-resolution XPS spectra of the Pt4f peaks of the Pt₂Si-richsample. Quantitative analysis of the XPS results indicated the formationof 72% of Pt₂Si. FIG. 9C shows the high-resolution XPS spectra of thePt4f peaks of the Pt₃Si-rich sample. Quantitative analysis of the XPSresults indicated the formation of 88% of Pt₃Si. FIG. 9D shows thehigh-resolution XPS spectra of the Pt4f peaks of the as-deposited sampleQuantitative analysis of the XPS results indicates the presence of puremetallic Pt.

FIGS. 10A and 10B illustrate the determination of the dominant diffusionspecies (DDS) during Pt₃Si formation in accordance with an exemplaryembodiment of the disclosed subject matter. FIG. 10A shows a schematicof an as-deposited sample composed of an a-Si film covered by Ptnanoparticles in accordance with an exemplary embodiment of thedisclosed subject matter, along with a TEM image. FIG. 10A further showsthe differences depending on whether Pt or Si is the DDS. FIG. 10B showsTEM images before (at 130° C.) and after (280° C.) Pt₃Si formation. Theoverall shape and size of the Pt and Pt₃Si nanoparticles is similar andindicates that Pt₃Si formation occurs through diffusion of a-Si into Pt.The electron diffraction pattern shows that Pt_(x)Si particles wereformed.

FIG. 11 shows a comparison of diffraction pattern intensitydistributions obtained from kinetically-limited and source-limitedsolid-state diffusion samples in accordance with an exemplary embodimentof the disclosed subject matter.

FIG. 12 shows TEM images of anti-phase boundaries (APBs) formed in Pt₃Siin accordance with an exemplary embodiment of the disclosed subjectmatter. Electron diffraction images of APB show single-crystaldiffraction spots.

FIG. 13 shows a comparison of sheet resistance measurements taken fromPt, Pt₃Si, and Pt₂Si samples fabricated in accordance with an exemplaryembodiment of the disclosed subject matter.

DETAILED DESCRIPTION

In accordance with one aspect of the disclosed subject matter, a methodfor forming a thin film comprising a metal silicide is provided. Themethod can include providing a substrate, depositing at least two thinfilm layers on the substrate, the at least two film layers comprising afirst layer comprising amorphous silicon and a second layer comprisingmetal contacting the first layer, and annealing the at least two filmlayers to form a metal silicide. In accordance with embodiments of thedisclosed subject matter, the method can be, for example, asource-limited method or a kinetically-limited method.

An exemplary embodiment of a method for forming a thin film comprising ametal silicide in accordance with the disclosed subject matter is shownin FIG. 1. A substrate can be provided at 102. The substrate can be anyobject, regardless of the shape, that be coated in a conformal mannerwith at least two film layers. The substrate can be, for example, awafer such as a silicon wafer. For example, FIG. 2A shows across-section view of a sample formed in accordance with an exemplaryembodiment of the disclosed subject matter. The substrate 202 in FIG. 2Ais a single crystal silicon wafer. In accordance with other embodimentsof the disclosed subject matter, the substrate can be a cylinder-shapedobject. The materials used for the substrate can be chosen asappropriate for the individual process. For example, the materials forthe substrate can be chosen such that they will not melt or sublimate atthe annealing temperature, which can vary based on the deposited filmlayers. The materials can also be chosen such that they will benon-reactive to the buffer layer at the annealing temperature.

In accordance with embodiments of the disclosed subject matter, adiffusion buffer 204 can be deposited on the substrate 202. Thediffusion buffer 204 can act to prevent diffusion of the metal and/orsilicon into the substrate during annealing. The diffusion buffer 204can be, for example, a layer of silicon nitride (Si₃N₄), a layer ofsilicon oxide, a layer of non-porous aluminum oxide (Al₂O₃), or a layerof aluminum nitride (A1N). In accordance with other embodiments, thebuffer can be any material that does not react with the metal oramorphous silicon at the annealing temperature. The diffusion buffer 204can be sputter deposited on the substrate 202.

With further reference to FIG. 1, at least two film layers can bedeposited on the substrate at 104. The at least two film layers caninclude a first layer comprising amorphous silicon and a second layercomprising metal. The first layer can be in contact with the secondlayer 208. For example, and as shown in FIG. 2A, the first layer 206comprising amorphous silicon can be deposited onto the substrate 202.The first layer 206 can be deposited directly onto the substrate 202, ormay be deposited onto the substrate via one or more intermediate layerssuch as diffusion buffer 204. The second layer 208 can then be depositedonto the first layer 206. The first and second layers 206 and 208 can bedeposited via sputter depositing, atomic layer deposition, thermalevaporation, chemical vapor deposition, molecular beam epitaxy, or otherthin film depositions techniques as known in the art.

In accordance with certain embodiments of the disclosed subject matter,a third film layer comprising silicon can be deposited onto the secondlayer 208. In accordance with other embodiments, additional alternatinglayers of amorphous silicon and metal can be deposited. Diffusion canoccur more uniformly from both directions with such arrangements.

The second layer 208 can be a layer of platinum. In accordance withother embodiments, the second layer 208 can include at least one oftitanium, nickel, tungsten, cobalt, molybdenum, and chromium.

With further reference to FIG. 1, the at least two layers can beannealed at 106. A cross-section view of a thin film including metalsilicide in accordance with an exemplary embodiment of the disclosedsubject matter is shown in FIG. 3A. The thin film in FIG. 3A is theresult of annealing the sample of FIG. 2A. The metal silicide layer 306can be platinum silicide. For example, the metal silicide layer caninclude Pt₃Si. In accordance with embodiments of the disclosed subjectmatter, the metal silicide layer can include at least about 40% Pt₃Si,at least about 45% Pt₃Si, at least about 50% Pt₃Si, at least about 55%Pt₃Si, at least about 60% Pt₃Si, at least about 65% Pt₃Si, at leastabout 70% Pt₃Si, at least about 74% Pt₃Si, at least about 75% Pt₃Si, atleast about 80% Pt₃Si, at least about 85% Pt₃Si, or at least about 88%Pt₃Si. For example, the metal silicide layer can include between about50% and about 60%, between about 55% and about 65%, between about 60%and about 70%, between about 65% and about 74%, between about 40% andabout 74%, between about 50% and about 65%, between about 50% and about74%, between about 60% and about 74%, between about 70% and about 74%Pt₃Si, between about 50% Pt₃Si and about 88% Pt₃Si, between about 60%Pt₃Si and about 85% Pt₃Si, between about 70% Pt₃Si and about 80% Pt₃Si,between about 70% Pt₃Si to about 85% Pt₃Si, between about 75% Pt₃Si toabout 88% Pt₃Si, or between about 80% Pt₃Si and about 88% Pt₃Si.

In accordance with embodiments of the disclosed subject matter, themethod can further include using at least one of a source-limited methodand a kinetically-limited method to tune a selected phase of the metalsilicide.

An exemplary embodiment of a source-limited method for forming a thinfilm comprising a metal silicide is shown in FIG. 4. A desired phase ofa metal silicide can be selected at 402. The desired phase of platinumsilicide can be, for example, PtSi, Pt₂Si, or Pt₃Si. The desired phaseof the metal silicide can be selected depending on the needs for aparticular application.

A ratio of an amount of metal to an amount of amorphous silicon can thenbe determined based on the desired phase at 404. Source-limited methodscan use the relative amounts of the starting materials (e.g., byprecisely controlling the precursor thin film layer thicknesses) topredetermine the achievable silicide stoichiometry after annealing. Forexample, a ratio of about 1:3 Pt:a-Si can be used to form PtSi, about1:1 can be used to form Pt₂Si, and about 1:2 can be used to form Pt₃Si.The ratio can be determined experimentally such as by trial and erroruntil a desired ratio is identified. Alternatively, the ratio can becalculated, e.g., by comparing the effective amount of moles of metal(e.g., Pt) and Si based on their respective densities. In accordancewith another embodiment, the ratio can be obtained from a third party.

A substrate can be provided at 406. The substrate can be a siliconwafer. At least two film layers can be deposited on the substrate at408. The at least two thin film layers can be deposited directly ontothe substrate, or can be deposited on the substrate via one or moreintermediate layers such as a diffusion barrier. The at least two filmlayers include at least a first layer comprising amorphous silicon and asecond layer comprising metal. The metal can be, for example, platinum,titanium, nickel, tungsten, cobalt, molybdenum, or chromium.

The at least two film layers include amorphous silicon and metal inamounts based on the selected ratio. For example, if the ratio is 1:1,amorphous silicon and metal can be deposited on the substrate in equalamounts. In accordance with one embodiment of the disclosed subjectmatter, the thickness of the layers deposited onto the substrate can bevaried to match the desired ratio. For example, if the selected ratio is1:1, a layer of amorphous silicon having a thickness of 100 nm can bedeposited on the substrate, and a layer of metal having a thickness of100 nm can be deposited onto the layer of amorphous silicon.

The at least two film layers can then be annealed to form the desiredphase of the metal silicide at 410. The metal silicide can be, forexample, platinum silicide, titanium silicide, nickel silicide, tungstensilicide, cobalt silicide, molybdenum silicide, or chromium silicide.The desired phase of platinum silicide can be, for example, PtSi, Pt₂Si,or Pt₃Si. In accordance with embodiments of the disclosed subjectmatter, the metal silicide layer can include at least about 40% Pt₃Si,at least about 45% Pt₃Si, at least about 50% Pt₃Si, at least about 55%Pt₃Si, at least about 60% Pt₃Si, at least about 65% Pt₃Si, at leastabout 70% Pt₃Si, or at least about 74% Pt₃Si. For example, the metalsilicide layer can include between about 50% and about 60%, betweenabout 55% and about 65%, between about 60% and about 70%, between about65% and about 74%, between about 40% and about 74%, between about 50%and about 65%, between about 50% and about 74%, between about 60% andabout 74%, or between about 70% and about 74% Pt₃Si.

An exemplary embodiment of a kinetically-limited method for forming athin film comprising a metal silicide is shown in FIG. 5. A desiredphase of a metal silicide can be selected at 502. The desired phase ofplatinum silicide can be, for example, PtSi, Pt₂Si, or Pt₃Si. Thedesired phase of the metal silicide can be selected depending on theneeds for a particular application.

A time-temperature regime can then be determined based on the desiredphase at 504. Kinetically-limited methods can use the annealing process(e.g., by precisely controlling the time-temperature regime) to obtain adesired stoichiometry and phase. For example, a sample including a layerof amorphous silicon and a layer of platinum can be heated at a firsttemperature to obtain Pt₃Si, at a second temperature higher than thefirst temperature to obtain Pt₂Si, or at a third temperature higher thanthe second temperature to obtain PtSi. For example, the firsttemperature can be approximately 200° C., the second temperature can beapproximately 300° C., and the third temperature can be approximately500° C. The time for which the sample is heated at the appropriatetemperature can also be varied. The time-temperature regime can bedetermined experimentally such as by trial and error until a desiredtime-temperature ratio is identified. Alternatively, thetime-temperature regime can be calculated or obtained from a thirdparty.

A substrate can be provided at 506. The substrate can be a siliconwafer. At least two film layers can be deposited on the substrate at508. The at least two thin film layers can be deposited directly ontothe substrate, or can be deposited on the substrate via one or moreintermediate layers such as a diffusion barrier. The at least two filmlayers include at least a first layer comprising amorphous silicon and asecond layer comprising metal. The metal can be, for example, platinum,titanium, nickel, tungsten, cobalt, molybdenum, or chromium.

The at least two film layers can then be annealed to form the desiredphase of the metal silicide at 510. The two film layers can be annealedusing the determined time-temperature regime to form the desired phaseof the metal silicide.

The metal silicide can be, for example, platinum silicide, titaniumsilicide, nickel silicide, tungsten silicide, cobalt silicide,molybdenum silicide, or chromium silicide. The desired phase of platinumsilicide can be, for example, PtSi, Pt₂Si, or Pt₃Si. In accordance withembodiments of the disclosed subject matter, the metal silicide layercan include at least about 40% Pt₃Si, at least about 45% Pt₃Si, at leastabout 50% Pt₃Si, at least about 55% Pt₃Si, at least about 60% Pt₃Si, atleast about 65% Pt₃Si, at least about 70% Pt₃Si, at least about 75%Pt₃Si, at least about 80% Pt₃Si, at least about 85% Pt₃Si, or at leastabout 88% Pt₃Si. For example, the metal silicide layer can includebetween about 50% and about 60%, between about 55% and about 65%,between about 60% and about 70%, between about 65% and about 75%,between about 40% and about 75%, between about 50% and about 65%,between about 50% and about 75%, between about 60% and about 75%,between about 70% and about 75% Pt₃Si, between about 50% Pt₃Si and about88% Pt₃Si, between about 60% Pt₃Si and about 85% Pt₃Si, between about70% Pt₃Si and about 80% Pt₃Si, between about 70% Pt₃Si to about 85%Pt₃Si, between about 75% Pt₃Si to about 88% Pt₃Si, or between about 80%Pt₃Si and about 88% Pt₃Si.

In accordance with another aspect, the disclosed subject matter providesa thin film comprising metal silicide. In accordance with oneembodiment, the disclosed subject matter provides a thin film comprisingmetal silicide formed by a process including providing a substrate,depositing at least two thin film layers on the substrate, the at leasttwo thin film layer comprising a first layer comprising amorphoussilicon and a second layer comprising metal contacting the first layer,and annealing the at least two thin film layers to form a metalsilicide.

In accordance with another embodiment, the disclosed subject matterprovides a thin film comprising a metal silicide formed by a processincluding identifying a desired phase of a metal silicide, determining aratio of an amount of a metal to an amount of amorphous silicon based onthe desired phase of the metal silicide, providing a substrate,depositing at least two thin film layers on the substrate, the at leasttwo thin film layers comprising a first layer comprising amorphoussilicon and a second layer comprising metal contacting the first layer,the at least two film layers comprising the amorphous silicon and themetal in amounts based on the determined ratio, and annealing the atleast two film layers to form the desired phase of the metal silicide.

In accordance with yet another embodiment, the disclosed subject matterprovides a thin film comprising a metal silicide formed by a processincluding identifying a desired phase of a metal silicide, determining atime-temperature regime based on the desired phase, providing asubstrate, depositing at least two thin film layers on the substrate,the at least two thin film layers comprising a first layer comprisingamorphous silicon and a second layer comprising metal contacting thefirst layer, and annealing the at least two film layers using thedetermined time-temperature regime to form the desired phase of themetal silicide.

In accordance with a further embodiment, the disclosed subject mattercan provide a thin film comprising a metal silicide formed by a processcomprising providing a first layer of amorphous silicon and a secondlayer of metal, and diffusing the amorphous silicon into the metal. Thediffusing can include annealing the first layer of amorphous silicon andthe second layer of metal.

Thin films formed according to the above-referenced processes can havehigher electrical conductivity than metal silicides formed using singlecrystal silicon while possessing similar mechanical properties. The thinfilms can also include anti-phase boundaries. Anti-phase boundaries canhinder dislocation motion and increase the yield strength of thematerial. In each of the aforementioned thin films, the thin film cancomprise at least one of a platinum silicide, a titanium silicide, anickel silicide, a cobalt silicide, a molybdenum silicide, or a chromiumsilicide.

In accordance with a further embodiment, the disclosed subject mattercan provide a thin film comprising Pt₃Si. The thin film can include atleast about 40% Pt₃Si, at least about 45% Pt₃Si, at least about 50%Pt₃Si, at least about 55% Pt₃Si, at least about 60% Pt₃Si, at leastabout 65% Pt₃Si, at least about 70% Pt₃Si, at least about 74% Pt₃Si, atleast about 75% Pt₃Si, at least about 80% Pt₃Si, at least about 85%Pt₃Si, or at least about 88% Pt₃Si. For example, the metal silicidelayer can include between about 50% and about 60%, between about 55% andabout 65%, between about 60% and about 70%, between about 65% and about74%, between about 40% and about 74%, between about 50% and about 65%,between about 50% and about 74%, between about 60% and about 74%,between about 70% and about 74% Pt₃Si, between about 50% Pt₃Si and about88% Pt₃Si, between about 60% Pt₃Si and about 85% Pt₃Si, between about70% Pt₃Si and about 80% Pt₃Si, between about 70% Pt₃Si to about 85%Pt₃Si, between about 75% Pt₃Si to about 88% Pt₃Si, or between about 80%Pt₃Si and about 88% Pt₃Si.

Thin films comprising metal silicides can be used in a wide variety ofapplications. For example, the thin films can be used as contactmaterials for nanoelectromechanical switches. The thin films can also beused in jet engines, plasmonic devices, lithium-ion batteries,field-emitters, and atomic force microscopy probes.

EXAMPLES

The Pt and a-Si films were sputter deposited in a Denton Vacuum Explorer14 sputterer (Denton Vacuum Inc, Moorestown, N.J.) with a purity of99.99% for both films. Pt was deposited in DC mode at 450 W and a-Si inAC mode at 230 W. The Pt nanoparticles were produced by coating an a-Sifilm with approx. 10 nm of Pt, which does not wet the surfacecompletely, forming nanoparticles instead of a continuous film.

An in situ heating TEM sample holder (Gatan Inc., Pleasanton, Calif.)was used to form Pt_(x)Si inside the TEM. The heating holder is equippedwith a thermocouple sensitive to ±1° C. All source-controlledsolid-state diffusion samples were subject to a similar annealingtreatment which included the heating up to 500° C. (30° C./min) andholding at 500° C. for 10 min. This was followed by heating up to 600°C. (30° C./min), holding at 600° C. for 10 min and rapid cooling to 50°C. (85° C./min). The kinetically-limited solid-state diffusion samplewas heated up under 30° C./min until 200° C. and rapidly quenched toconserve the Pt₃Si phase. Subsequently, the sample was heated up to 300°C. (30° C./min) to form Pt₂Si followed by rapid quenching. Finally, thesample was heated up to 500° C. (30° C./min) to form PtSi andsubsequently cooled down.

All TEM experiments were performed using a JEOL 2100 thermionic emissionsource TEM (JEOL Ltd., Tokyo, Japan). An accelerating voltage of 200 kVwas used with a beam current of 106 μA resulting in a current density of275 pA/cm².

The chemistry of the near-surface region was investigated by X-rayphotoelectron spectroscopy (XPS) using a customized XPS spectrometer (VGScienta AB, Uppsala, Sweden) 17. XPS analyses were performed using amonochromatic Al Kα source (photon energy: 1486.6 eV). The residualpressure in the analysis chamber was constantly less than 1·10⁻⁸ Torr.The spectrometer was calibrated according to ISO 15472:2001 with anaccuracy of ±0.05 eV. Survey and high resolution spectra were acquiredin constant-analyzer-energy mode with the pass energies of 200 eV and100 eV, respectively. The full width at half-maximum (FWHM) of thepeak-height for the high-resolution Ag 3d_(5/2) signal of asputter-cleaned Ag sample was 0.57 eV. The spectra were processed usingCasaXPS software (v.2.3.16, Case Software Ltd., Wilmslow, Cheshire,U.K.). Background subtraction was performed using the Shirley-Sherwoodmethod. The quantitative evaluation of XPS data was based on integratedintensity using a first-principles model and applying Powell's equation.The inelastic mean free path was calculated using the TPP-2M formula.Curve synthesis for the Pt 4f peaks was performed constraining theintegrated intensity ratio of these two signals to 3:4 and their energyseparation to 3.33 eV. The reference energies for Pt 4f7/2 peaks are71.05 eV, 71.55 eV, 72.18 eV, and 72.75 eV for Pt, Pt₃Si, Pt₂Si, andPtSi respectively.

Samples of varying Pt:a-Si film thickness ratios were fabricated to formPt_(x)Si films of varying stoichiometry using the source-limitedsolid-state diffusion approach and to assess their crystal structuresand formation sequence. To achieve this, PELCO 50 nm silicon nitride(Si₃N₄) support TEM grids (Ted Pella Inc., Redding, Calif.) weresputter-coated with thin layers of a-Si and Pt, whose thicknesses wereoptimized to obtain nearly pure phases of Pt₃Si, Pt₂Si and PtSi uponannealing. The Pt and a-Si depositions were conducted sequentially inthe same deposition system under maintained vacuum. This procedureminimizes contaminant adsorption between the layers and the oxidation ofthe a-Si—both inhibiting factors for silicidation. The Pt/a-Si coatedSi₃N₄-support TEM grids were then removed from the deposition system andsubsequently characterized using TEM (JOEL 2100 TEM, JEOL Ltd., Tokyo,Japan). An in situ heating TEM sample holder (Gatan Inc., Pleasanton,Calif.) was used to anneal the samples inside the TEM while continuouslyrecording the electron diffraction pattern, which allowed the formationsequence of the different Pt_(x)Si phases to be determined. All sampleswere subjected to similar annealing treatments that included a 10 minhold at 500° C. followed by a 10 min hold at 600° C. High resolutionelectron diffraction studies of the as deposited and annealed samplesallowed the determination of PtxSi crystal structure.

The results from the source-limited solid-state diffusion experimentsare shown in FIG. 6. The Pt:a-Si film thickness ratios were chosen to be1:3 (Pt:a-Si=1:3) to form PtSi, 1:1 (Pt:a-Si=1:1) for Pt₂Si, and 2:1(Pt:a-Si=2:1) for Pt₃Si. FIG. 6a shows high-resolution electrondiffraction patterns of one of the as-deposited Pt/a-Si samples (similarpatterns were obtained for all as-deposited Pt/a-Si samples) and theformed Pt_(x)Si films. The diffraction pattern of the as depositedPt/a-Si film (top-right quadrant in FIG. 6a ) matches the theoreticalpattern for Pt, as the amorphous structure of the a-Si film and Si₃Ni₄membrane do not produce any diffraction rings. All theoreticaldiffraction patterns have been calculated using the theoretical crystalstructures of Pt and PtxSi phases and CrystalMaker software(CrystalMaker Software Ltd., Oxfordshire, United Kingdom). Thediffraction rings shown in the bottom-right quadrant in FIG. 6a matchthe theoretical diffraction spots for monoclinic Pt₃Si, whichdemonstrates that the Pt:a-Si=2:1 sample led to the formation of Pt₃Siupon annealing. The bottom-left quadrant in FIG. 6a shows thediffraction pattern of the Pt:a-Si=1:1 sample upon annealing, whichmatches the theoretical diffraction pattern for tetragonal Pt₂Si. Thetop-left quadrant in FIG. 6a shows the diffraction pattern that wasobtained after annealing the Pt:a-Si=1:3 sample. The diffraction ringsmatch with the theoretical diffraction pattern of orthorhombic PtSi.FIG. 6b summarizes the formation process of the Pt₃Si, Pt₂Si and PtSifilms. The formation process of all three Pt_(x)Si stoichiometriesstarts with the formation of the Pt₃Si phase. This is the only phasethat was observed for the Pt:a-Si=2:1 sample and was stable throughoutthe complete annealing procedure. However, considerable grain-growthoccurred as a result of the low Pt₃Si formation temperature of 200° C.and the continued heat treatment up to 600° C. The grain growth resultsin the appearance of high-intensity diffraction spots in thebottom-right quadrant of FIG. 6a . The Pt₂Si formation followed thePt₃Si formation at approximately 300-325° C. for the Pt:a-Si=1:1 andPt:a-Si=1:3 samples. This phase remained stable for the Pt:a-Si=1:1sample, whereas PtSi was formed around 400-425° C. in the Pt:a-Si=1:3sample.

The annealed TEM grids were additionally characterized usinghigh-resolution angle resolved X-ray photoelectron spectroscopy (XPS) todetermine the stoichiometry of the as deposited Pt and the formedPt_(x)Si phases. The surface characterization of as-deposited PT andformed Pt₃Si on TEM rides using source-limited solid state diffusion isshown in FIG. 7. The high-resolution XPS spectra of as-deposited Pt isdisplayed in FIG. 7d . The Pt4f_(7/2) peak position (71.05 eV) andlineshape are characteristic for metallic platinum, thus confirming theTEM diffraction pattern displayed in FIG. 6a (top right quadrant). FIG.7c shows the high-resolution XPS spectra of the annealed Pta-Si=2:1sample, which displayed Pt₃Si-like diffraction behavior in FIG. 6a(bottom-right quadrant). Quantitative analysis (reported in the insetsin FIG. 7) determined that this sample consists of 74% Pt₃Si and 26%metallic Pt. FIG. 7b shows the high-resolution XPS spectra of theannealed Pta-Si=1:1 sample, which displayed Pt₂Si-like diffractionbehavior in FIG. 6a (bottom-left quadrant). Quantitative XPS analysisshowed that this sample consists of 70% Pt₂Si phase, while the remaining30% are a mixture between metallic Pt, Pt₃Si and PtSi (7% Pt, 14% Pt₃Si,9% PtSi). The high-resolution XPS spectra of the annealed Pt:a-Si=1:3sample is displayed in FIG. 7a . This sample is composed of 92% PtSiphase (remainder: 2% Pt, 1% Pt₃Si, 5% Pt₂Si), which matches well withthe PtSi diffraction pattern this sample exhibited in FIG. 6a (left-topquadrant). The quantitative XPS analysis confirmed the Pt_(x)Sistoichiometries that have been determined using in situ TEM electrondiffraction. The overall results obtained from in situ TEM and XPSexperiments on samples of different Pt:a-Si film thickness ratiosconfirm that the stoichiometry of Pt_(x)Si thin films can be preciselytuned using source-controlled solid-state diffusion.

Samples of Pt:a-Si thickness ratio of 1:3 were produced to test if theprecise control of the temperature-time regime during silicidation canbe used to tune the Pt_(x)Si stoichiometry (i.e. kinetically-limitedsolid-state diffusion). This Pt:a-Si thickness ratio will ultimatelylead to PtSi formation, while passing the Pt₃Si and Pt₂Sistoichiometries (as seen in FIG. 6b ). FIG. 8 summarizes the resultsfrom the kinetically-limited solid-state diffusion experiments. FIG. 8ashows high-resolution electron diffraction images recorded at roomtemperature and after the annealing process has been halted at specifictemperatures to produce highly selective Pt_(x)Si phases. Theas-deposited sample, which exhibited a Pt electron diffraction pattern(see top-tight quadrant in FIG. 8a ), was heated up to 200° C. to formPt₃Si and then quickly cooled down to retain this phase at roomtemperature (see bottom-right quadrant in FIG. 8a ). The Pt₃Si film wassubsequently heated up to 300° C. to form Pt₂Si and quickly cooled downto retain this phase at room temperature (see bottom-left quadrant inFIG. 8a ). Finally, the formed Pt₂Si film was heated up to 500° C. sothat PtSi is formed and subsequently cooled down (see top-left quadrantin FIG. 8a ). Comparison of the kinetically-limited diffraction patterns(FIG. 8a ) with those from the source-limited experiments (FIG. 6a )show that the peak positions and intensity ratios are very similar whichindicates a similar composition of the Pt_(x)Si phases forkinetically-limited and source-limited experiments (see FIG. 11). FIG.8a also shows that all formed phases exhibit minimal grain growth.Dark-field TEM results also indicated that the formed Pt_(x)Si films arenanocrystaline. FIG. 8b summarizes the kinetically-limited Pt_(x)Siformation experiments and shows that the stoichiometry of Pt_(x)Si filmswas precisely tunable by controlling the temperature-time regime duringthe annealing. The in situ TEM experiments presented in FIG. 8 provethat kinetically-controlled solid-state diffusion is a powerful way totune the Pt_(x)Si stoichiometry.

The surface characterization of as-deposited Pt and formed Pt₃Si on TEMrides using source-limited solid state diffusion is shown in FIG. 9. Thehigh-resolution XPS spectra of as-deposited Pt is displayed in FIG. 9d .FIG. 9c shows the high-resolution XPS spectra of the sample afterannealing at 200° C. Quantitative analysis (reported in the insets inFIG. 9) determined that this sample consists of 88% Pt₃Si. FIG. 9b showsthe high-resolution XPS spectra of the sample after annealing at 300° C.Quantitative XPS analysis showed that this sample consists of 72% Pt₂Siphase. The high-resolution XPS spectra of the sample after annealing at500° C. is displayed in FIG. 9a . This sample is composed of 91% PtSiphase.

Both source-limited and kinetically-limited solid-state diffusionexperiments (FIG. 6, FIG. 8) were able to produce highly selective Pt₃Sifilms, which have not been experimentally reported before or observedduring Pt/sc-Si experiments. The well-studied Pt/sc-Si system ischaracterized by the specific Pt_(x)Si formation sequence of Ptdiffusing into sc-Si around 250-300° C. to form the intermediate Pt₂Siphase. After all Pt is consumed, sc-Si will diffuse into Pt₂Si at300-450° C. to form the thermodynamically stable PtSi phase. Theamorphous nature of the a-Si film used in accordance with exemplaryembodiments of the disclosed subject matter alters the diffusionbehavior compared to sc-Si, which leads to the formation of the Pt₃Siphase. Without wishing to be bound to any particular theory, it isbelieved that the Pt₃Si phase is formed due to the fact that in thePt/a-Si system, a-Si acts initially as the dominant diffusing species(DDS) in contrast to the Pt/sc-Si system where Pt is initially the DDS.The larger “free space” in a-Si compared to sc-Si leads to reducedactivation energies for a-Si self-diffusion of up to 27% compared tothose for sc-Si. This means that a-Si becomes mobile at lowertemperatures than sc-Si. In addition to the increased mobility of a-Si,the Pt diffusivity is drastically reduced in a-Si compared to sc-Si. Pthas a diffusion coefficient at 500° C. of ˜5·10⁻¹⁸ m²/s in a-Si, whereasthe diffusion coefficient of Pt in sc-Si is ˜5·10⁻¹⁶m²/s, which meansthat Pt diffuses 2 orders of magnitude faster in sc-Si than in a-Si.Both, the increased a-Si and reduced Pt diffusivity contribute to thea-Si diffusion into Pt and subsequent formation of Pt₃Si for the Pt/a-Sisystem. The observed Pt₃Si formation can also be understood by the highdegree of resemblance between the Pt and Pt₃Si crystal structures,including the lattice parameter. The Pt₃Si unit cell can be imagined asa Pt unit cell with the corner atoms replaced by Si. The ease oftransformation between Pt and Pt₃Si without the need of massive atomicrestructuring is thought to be the reason for such low temperaturemetamorphosis. To demonstrate that a-Si is the diffusing species insteadof Pt and that this diffusion leads to the formation of Pt₃Si, we coateda Si₃N₄-support TEM grid with a-Si and, subsequently, with Ptnanoparticles of ˜10 nm diameter (as shown in FIG. 10a ). FIG. 10a showsa schematic and TEM image of the as-deposited sample together with thetheoretical pathways leading to the formation of Pt₃Si, i.e., thediffusion of Pt in a-Si or the diffusion of a-Si in Pt. If Pt is theDDS, then the Pt nanoparticles will diffuse into the underlying a-Silayer and a TEM Z-contrast image will show significant broadening of thePt containing regions. If, on the other hand, a-Si is the diffusingspecies as proposed here, then the general shape of the Pt nanoparticlesshould be conserved during Pt₃Si formation as only a small amount ofa-Si is diffusing into the nanoparticles. FIG. 10b shows an area of thesample with several Pt nanoparticles before (at 130° C.) and after (at280° C.) Pt₃Si formation. The overall shape and size of thenanoparticles do not undergo any significant changes due to Pt₃Siformation. This result confirms that a-Si is the DDS, and that itdiffuses into Pt to form Pt₃Si, which means that the occurrence forPt₃Si in a-Si-based silicidation experiments is entirely due to the highdiffusivity of a-Si, and that such films are not attainable using sc-Si.

A comparison of diffraction pattern intensity distributions obtainedfrom the kinetically-limited and source-limited solid-state diffusionsamples is shown in FIG. 11. The peak locations and intensity ratios aresimilar for both methods and indicate that the kinetically-limitedsamples possess similar compositions as the source-limited samples. Thetheoretical peaks positions of the strongest peaks are indicated by thedownward arrows.

The Pt₃Si films resulting from Pt/a-Si annealing have several intriguingadvantages over PtSi films that result from Pt/sc-Si. Most notably,Pt₃Si possesses similar mechanical properties as PtSi (˜50% greaterhardness and modulus compared to Pt), while possessing a higherelectrical conductivity. The increase in hardness in Pt₃Si can also beunderstood from our source-limited heating experiments where we observenot only grain coarsening but also formation of anti-phase boundaries(APBs) which are two dimensional defects commonly found in L12 typecrystal structures. TEM images of the APBs formed in Pt3Si are shown inFIG. 12. These APBs will hinder dislocation motion and thereby plasticdeformation.

Measurements taken from XPS experiments on PtSi thin film samplesfabricated according to an exemplary embodiment of the herein disclosedmethods were used to determine the density of states (DOS) at the FermiLevel, as shown below in Table 1. The measurements demonstrate that theherein disclosed methods can be employed to tailor the electronicstructure of metal thin films towards specific applications, such as,for example and without limitation, high conductive applications. Acomparison of sheet resistance (Ω/square) determined from measurementstaken in XPS experiments on PtSi thin film samples produced by oneembodiment of the herein disclosed subject matter is shown in FIG. 13 tofurther demonstrate that the electronic structure of metal thin filmscan be tailored to produce desirable properties, such as, for example,high conductivity. With reference to the example of FIG. 13, sheetresistance, or in-plan electrical resistance, is inversely correlatedwith electrical conductivity, such that a low sheet resistance reflectsa correspondingly high electrical conductivity. The measurements of FIG.13 illustrate a relatively low sheet resistance associated with thePt₃Si phase as compared to the PtSi phase, indicating high conductivity,which is confirmed in the XPS measurements of Table 1, below.Accordingly, the Pt_(x)Si thin film phases fabricated using the hereindisclosed methods provide advantages over the conventional PtSi thinfilm phase, which suffers from low conductivity due to its having a lowDOS at the Fermi Level, as shown by way of example in Table 1 and inrelation to its sheet resistance depicted in FIG. 13.

TABLE 1 Normalized Carrier Density XPS Pt 1 Pt₃Si 0.59 Pt₂Si 0.20 PtSi0.17

The measurements illustrated in FIG. 13 further demonstrate that thePt_(x)Si thin film phases fabricated using the herein disclosed methodsare superior to conventional PtSi thin films introduced with dopants.For example, in certain applications where PtSi is a desired material,such as in the semiconductor industry for contacting the source, drain,and/or gate to CMOS field effect transistors, conventional techniques toincrease the DOS can include introducing dopants into the PtSi thinfilm. These dopants can include, for example, titanium or other metals.Although the addition of titanium to PtSi was found in prior experimentsto increase the DOS from about 17% of the DOS of Pt to about 30% of theDOS of Pt, the titanium impurities decreased the carrier mobility of thethin film, thereby lowering its conductivity. Thus, the Pt_(x)Si phasesproduced from the techniques herein disclosed, and particularly thePt₃Si phase, have advantages over conventional metal thin films andmethods of formation and can be used in applications that require highelectrical conductivity, such as for example, in semiconductortechnologies.

The presently disclosed subject matter is not to be limited in scope bythe specific embodiments herein. Indeed, various modifications of thedisclosed subject matter in addition to those described herein willbecome apparent to those skilled in the art from the foregoingdescription and the accompanying figures.

1. A thin film comprising Pt₃Si.
 2. The thin film of claim 1, whereinthe thin film is selected from a group consisting of at least 40% Pt₃Si,at least 45% Pt₃Si, at least 50% Pt₃Si, at least 55% Pt₃Si, at least 60%Pt₃Si, at least 65% Pt₃Si, at least 70% Pt₃Si, at least 74% Pt₃Si, atleast about 75% Pt₃Si, at least about 80% Pt₃Si, at least about 85%Pt₃Si, and at least about 88% Pt₃Si.
 3. The thin film of claim 1,wherein the thin film comprises a film formed by at least one of asource-limited method and a kinetically-limited method.
 4. A thin filmincluding a metal silicide having a phase, comprising: a substrate; afirst layer comprising a first amount of amorphous silicon deposited onsaid substrate; and a second layer comprising a second amount of a metaldeposited on the first layer; wherein a ratio of the amount of theamorphous silicon and the amount of the metal correspond to the phaseupon annealing thereof; and
 5. The thin film of claim 4, wherein themetal comprises at least one of platinum, titanium, nickel, tungsten,cobalt, molybdenum, and chromium.
 6. The thin film of claim 4, whereinthe metal silicide comprises platinum silicide.
 7. The thin film ofclaim 6, wherein the platinum silicide is Pt₃Si.
 8. The thin film ofclaim 4, wherein the annealing comprises: annealing the at least twofilm layers using a time-temperature regime to form the desired phase ofthe metal silicide.
 9. A nanoelectromechanical switch comprising thethin film of claim
 1. 10. A jet engine comprising the thin film ofclaim
 1. 11. A device comprising the thin film of claim 1, the deviceselected from a group consisting of a plasmonic device, a lithium-ionbattery, a field emitter, and an atomic force microscopy probe.